In the semiconductor integrated circuit industry, the cost of individual integrated circuit chip die is continuing to decrease in comparison to IC package costs. Consequently, it is becoming more important to perform many IC test and evaluation steps while the die are still in the wafer, rather than after the relatively expensive packaging steps have been performed.
Increasingly, in IC processing, semiconductor wafers are subjected to a series of test and evaluation steps. For each step, the wafer is held in a stationary position at a process station where the process is performed. For many processes, it is important that the wafer be held extremely flat. For example, circuit testing is typically performed over a wide temperature range to temperature screen the ICs before assembly into a package. The wafer is typically held on a vacuum platform of a host test machine such as a probing station which electrically tests the circuits on the wafer. The prober includes a group of electrical probes which, in conjunction with a tester, apply predetermined electrical excitations to various predetermined portions of the circuits on the wafer and sense the circuits' responses to the excitations. To ensure that proper electrical contacts are made and to ensure that the mechanical load applied by the probes to the wafer is known and uniform, it is important to keep the wafer extremely flat.
In a typical prober system, the wafer is mounted on the top surface of a wafer chuck, which is held at its bottom surface to a support structure of the prober. A vacuum system is typically connected to the chuck. A series of channels or void regions in communication with the top surface of the chuck conduct the vacuum to the wafer to hold it in place on the top surface of the chuck. The prober support structure for the chuck is then used to locate the wafer under the probes as required to perform the electrical testing on the wafer circuits.
To allow for temperature screening of the wafer circuits, the chuck can also include a heater for heating the wafer to a desired temperature and a heat sink for cooling the wafer as needed. The prober system in conjunction with the chuck can then be used to analyze performance of the wafer circuits at various temperatures within a predetermined temperature range.
Conventional wafer chucks are formed from multiple components fastened together. For example, a typical chuck can include a lower plate or support for mounting to the prober, a heat sink over the lower plate, a heater over the heat sink and an upper plate or support assembly on which the wafer can be held, the upper plate including the vacuum channels used to conduct the vacuum to the top surface. In conventional chucks, all of these layers are typically held together by bolts, rivets, etc., or other rigid, inflexible mechanical fastening means. Furthermore, the chuck is typically held to the base of the host machine by similar rigid means.
These conventional means for holding the chuck together and holding the chuck to the base introduce mechanical stresses into the chuck structure. When the chuck is subjected to variations in temperature, these stresses tend to cause the chuck to deform, resulting in a loss of flatness of the wafer. The non-flat upper surface of the wafer can introduce inaccuracies into the circuit performance measurements performed by the prober.
The deformation in the chuck is typically caused by different chuck layers having different thermal expansion coefficients, such that, over temperature, different layers will experience different thermal expansion forces. Because the chuck layers are held together rigidly, the difference in forces causes the chuck to warp. As the chuck deforms, expansion forces build-up in the chuck. In most chucks, the clamping forces holding the layers together are sufficient to resist relative radial movement between the layers, and the warp increases. In some chucks, the clamping forces are such that, periodically, they are overcome by the expansion forces, and layers move rapidly in a jerking motion relative to each other to relieve the built-up stresses. This rapid "popping" motion is highly unpredictable and can introduce substantial wafer shape and/or location errors. Also, because the clamping forces are so high in these systems, the chuck layers are not relieved all the way back to a zero-expansion condition. So, in general, there is always some undetermined amount of deformation in the chuck over temperature.
It will be appreciated that these effects caused by the conventional mechanically constrained chuck assembly are magnified for larger diameter chucks. That is, the stresses introduced in clamping or bolting together a large diameter chuck are greater than those introduced in assembling a small diameter chuck. Larger chucks therefore tend to deform more over temperature than do smaller chucks. Therefore, using conventional wafer chuck techniques, it is becoming increasingly more difficult to hold wafers flat over temperature as wafer diameters continue to increase.
Conventional wafer chucks used for temperature cycling are typically mounted on the prober support structure in a manner which provides for good thermal conduction between the chuck and the prober support structure. In these systems, large amounts of energy dedicated to temperature cycling of the wafer can be lost in the form of heat flow between the prober and the chuck. Also, temperature variations in the prober support structure can cause spatial shifts in the wafer which can cause inaccuracies in the prober circuit testing.